17.1. References

17.1.1. Overview

On the Internet, resources are identified using IRIs
(Internationalized Resource Identifiers). For
example, an SVG file called someDrawing.svg located at
http://example.com might have the following IRI:

http://example.com/someDrawing.svg

An IRI can also address a particular element within an XML
document by including an IRI fragment
identifier as part of the IRI. An IRI which includes an
IRI fragment identifier consists of an optional base IRI, followed by a "#" character,
followed by the IRI fragment identifier. For example, the
following IRI can be used to specify the element whose ID is
"Lamppost" within file someDrawing.svg:

http://example.com/someDrawing.svg#Lamppost

17.1.2. IRIs and URIs

Internationalized Resource Identifiers (IRIs) are a more generalized
complement to Uniform Resource Identifiers (URIs). An IRI is a sequence
of characters from the Universal Character Set [UNICODE].
A URI is constructed from a much more restricted set of characters. All URIs are
already conformant IRIs. A mapping from IRIs to URIs is defined by
the IRI specification, which means that IRIs can be used instead of URIs
in XML documents, to identify resources. IRIs can be converted to URIs
for resolution on a network, if the protocol does not support IRIs
directly.

Previous versions of SVG, following XLink, defined an IRI reference type
as a URI or as a sequence of characters which must result in an IRI after a
particular escaping procedure was applied. The escaping procedure was repeated in the
XLink 1.0 specification [XLINK], and in the
W3C XML Schema Part 2: Datatypes specification [SCHEMA2].
This copying introduced the possibility of error and divergence, but was done
because the IRI specification was not yet standardized.

In this specification, the correct term IRI is used for this "URI or sequence of characters
plus an algorithm" and the escaping method, which turns IRIs into URIs, is defined by reference to the
IRI specification [RFC3987],
which has since become an IETF Proposed Standard. Other W3C specifications are
expected to be revised over time to remove these duplicate descriptions of the
escaping procedure and to refer to IRI directly.

17.1.3. Syntactic forms: IRI and FuncIRI

IRIs are used in the ‘xlink:href’ attribute. Some attributes
allow both IRIs and text strings as content. To disambiguate a text string from
a relative IRI, the functional notation
<FuncIRI> is used. This is simply
an IRI delimited with a functional notation. Note: For
historical reasons, the delimiters are "url(" and ")", for compatibility with
the CSS specifications. The FuncIRI form is used in presentation attributes.

SVG makes extensive use of IRI references, both absolute and relative,
to other objects. For example, to fill a rectangle with a linear
gradient, you first define a ‘linearGradient’ element and give
it an ID, as in:

<linearGradient xml:id="MyGradient">...</linearGradient>

You then reference the linear gradient as the value of the
‘fill’ property for the rectangle, as in the following example:

<rect fill="url(#MyGradient)"/>

SVG supports two types of IRI references:

local IRI references, where the IRI reference does not contain an
<absoluteIRI> or
<relativeIRI> and thus only contains
a fragment identifier (i.e.,
#<elementID> or
#xpointer(id<elementID>))

17.1.4. Processing of IRI references

The following rules apply to the processing of IRI references:

IRI references to nodes that do not exist shall be treated as
invalid references.

IRI references to elements which are inappropriate
targets for the given reference shall be treated as invalid
references (see list below for appropriate targets). For
example, the ‘clip-path’ property can only refer to
‘clipPath’ elements. The property setting
clip-path:url(#MyElement) is an
invalid reference if the referenced element is not a ‘clipPath’.

The following rules apply to the processing of invalid IRI references:

An invalid local IRI reference (i.e., an invalid references to a node
within the current document) represents an error (see
Error processing), apart from the
‘xlink:href’ attribute on the ‘a’ element and the properties
that allow for backup values in the case where the IRI reference is invalid
(see ‘fill’ and ‘stroke’).

17.1.5. IRI reference attributes

IRI references are normally specified with an
‘href’ attribute in the XLink
[XLink] namespace. For example, if the
prefix of 'xlink' is used for attributes in the XLink namespace, then the
attribute is
specified as ‘xlink:href’. The value of this
attribute forms a reference for the desired resource (or secondary resource, if
there is a fragment identifier).

If the protocol, such as HTTP, does not support IRIs directly,
the IRI is converted to a URI by the SVG implementation, as described
in section 3.1 of the IRI specification [RFC3987].

Because it is impractical for any application to check that
a value is an IRI reference, this specification follows the lead
of the IRI Specification
in this matter and imposes no such conformance testing
requirement on SVG applications.

Identifies the type of XLink being used. In SVG 1.1, only
simple links are available.
Links are simple links by default,
so the attribute xlink:type="simple"
is optional and may be omitted on simple links.
Refer to the XML Linking Language (XLink) [XLINK].

An optional IRI reference that identifies some resource that describes
the intended property. The value must be an IRI reference as
defined in [RFC3987],
except that if the IRI scheme used is allowed to have
absolute and relative forms, the IRI portion must be
absolute. When no value is supplied, no particular role
value shall be inferred.
Refer to the XML Linking Language (XLink) [XLINK].

An optional IRI reference that identifies some resource that describes
the intended property. The value must be an IRI reference as
defined in [RFC3987],
except that if the IRI scheme used is allowed to have
absolute and relative forms, the IRI portion must be
absolute. When no value is supplied, no particular role
value shall be inferred. The arcrole
attribute corresponds to the [RDF-PRIMER] notion
of a property, where the role can be interpreted as stating
that "starting-resource HAS arc-role ending-resource." This
contextual role can differ from the meaning of an ending
resource when taken outside the context of this particular
arc. For example, a resource might generically represent a
"person," but in the context of a particular arc it might
have the role of "mother" and in the context of a different
arc it might have the role of "daughter."
Refer to the XML Linking Language (XLink) [XLINK].

The title attribute shall be used to describe the meaning of
a link or resource in a human-readable fashion, along the
same lines as the role or arcrole attribute. A value is
optional; if a value is supplied, it shall contain a
string that describes the resource. In general it is preferable
to use a ‘title’ child element
rather than a ‘title’ attribute. The use of this
information is highly dependent on the type of processing
being done. It may be used, for example, to make titles
available to applications used by visually impaired users,
or to create a table of links, or to present help text that
appears when a user lets a mouse pointer hover over a
starting resource.
Refer to the XML Linking Language (XLink) [XLINK].

This attribute is provided for backwards compatibility with SVG
1.1. It provides documentation to XLink-aware processors. In
case of a conflict, the target attribute has priority, since it can
express a wider range of values.
Refer to the XML Linking Language (XLink) [XLINK].

In all cases, for compliance with either the "Namespaces in XML 1.0" or the "Namespaces in XML 1.1"
Recommendation [XML-NS10][XML-NS],
an explicit XLink namespace declaration must be provided whenever
one of the above XLink attributes is used within SVG content.
One simple way to provide such an XLink namespace declaration
is to include an ‘xmlns’ attribute
for the XLink namespace on the ‘svg’ element for content that uses
XLink attributes. For example:

17.2. Links out of SVG content: the ‘a’ element

SVG provides an ‘a’ element, to indicate links (also known
as hyperlinks or Web links). The ‘a’ element may
contain any element that its parent may contain, except itself.

SVG uses XLink ([XLink])
for all link definitions. SVG 1.1 only requires
that user agents support XLink's notion of
simple links.
Each simple link associates exactly two resources, one
local and one remote, with an arc going from the former to the latter.

A simple link is defined for each separate rendered element contained
within the ‘a’ element; thus, if the ‘a’ element contains
three ‘circle’ elements, a link is created for each circle. For
each rendered element within an ‘a’ element, the given rendered
element is the local resource (the source anchor for the link).

The remote resource (the destination for the link) is defined by
an IRI specified by the ‘xlink:href’ attribute on the ‘a’
element. The remote resource may be any Web resource (e.g., an image, a video
clip, a sound bite, a program, another SVG document, an HTML document, an
element within the current document, an element within a different document, etc.).
By activating these links (by clicking with the mouse, through keyboard
input, voice commands, etc.), users may visit these resources.

This attribute provides documentation to XLink-aware processors. If
target="_blank" then use
xlink:show="new" else use
'replace'. In
case of a conflict, the target attribute has priority, since it can
express a wider range of values.
Refer to the XML Linking Language (XLink) [XLINK].

This attribute provides documentation to XLink-aware processors that
an application should traverse from the starting
resource to the ending resource only on a post-loading
event triggered for the purpose of traversal.
Refer to the XML Linking Language (XLink) [XLINK].

This attribute should be used when there are multiple possible targets for
the ending resource, such as when the parent document is a
multi-frame HTML or XHTML document. This attribute specifies the
name or portion of the target window, frame, pane, tab, or other
relevant presentation context (e.g., an HTML or XHTML frame, iframe, or object element) into
which a document is to be opened when the link is activated:

_replace

The current SVG image is replaced by the linked content in the
same rectangular area in the same frame as the current SVG image.

_self

The current SVG image is replaced by the linked content in the
same frame as the current SVG image. If the attribute is not specified,
'_self' is assumed.

_parent

The immediate frameset parent of the SVG image is replaced by the
linked content.

_top

The content of the full window or tab, including any frames, is
replaced by the linked content

_blank

A new un-named window or tab is requested for the display of the
linked content. If this fails, the result is the same as _top

Specifies the name of the frame, pane, or other relevant
presentation context for display of the linked content. If this
already exists, it is re-used, replacing the existing content. If
it does not exist, it is created (the same as '_blank', except that
it now has a name).

Note: The value '_new' is not a legal value for target (use '_blank').

17.3. Linking into SVG content: IRI fragments and SVG views

Because SVG content often represents a picture or drawing
of something, a common need is to link into a particular
view of the document, where a view indicates
the initial transformations so as to present a closeup of a particular
section of the document.

17.3.1. SVG fragment identifiers

To link into a particular view of an SVG document, the IRI fragment
identifier needs to be a correctly formed SVG
fragment identifier. An SVG fragment identifier defines the
meaning of the "selector" or "fragment identifier" portion of IRIs that
locate resources of MIME media type "image/svg+xml".

An SVG fragment identifier can come in two forms:

Shorthand bare name form of addressing (e.g.,
MyDrawing.svg#MyView). This form of
addressing, which allows addressing an SVG element by its ID, is compatible
with the fragment addressing mechanism for older versions of HTML.

An SVG view specification
(e.g., MyDrawing.svg#svgView(viewBox(0,200,1000,1000))).
This form of addressing specifies the desired view of the
document (e.g., the region of the document to view, the
initial zoom level) completely within the SVG fragment
specification. The contents of the SVG view specification are
the five parameter specifications, viewBox(...),
preserveAspectRatio(...),
transform(...),
zoomAndPan(...) and
viewTarget(...), whose parameters have the
same meaning as the corresponding attributes on a ‘view’ element, or,
in the case of transform(...), the same
meaning as the corresponding attribute has on a
‘g’ element).

ViewBoxParams corresponds to the
parameter values for the ‘viewBox’ attribute on the ‘view’
element. For example, viewBox(0,0,200,200).

AspectParams corresponds to the
parameter values for the ‘preserveAspectRatio’ attribute on the
‘view’ element. For example,
preserveAspectRatio(xMidYMid).

TransformParams corresponds to the
parameter values for the ‘transform’ property that is available on
many elements. For example, transform(scale(5)).

ZoomAndPanParams corresponds to the
parameter values for the ‘zoomAndPan’ attribute on the ‘view’
element. For example, zoomAndPan(magnify).

ViewTargetParams corresponds to the
parameter values for the ‘viewTarget’ attribute on the ‘view’
element. For example, viewTarget(MyElementID).

Spaces are not allowed in fragment specifications; thus, commas
are used to separate numeric values within an SVG view specification
(e.g., #svgView(viewBox(0,0,200,200)))
and semicolons are used to separate attributes (e.g.,
#svgView(viewBox(0,0,200,200);preserveAspectRatio(none))).

Semicolons used to separate 'SVGViewAttribute' in SVG fragments may be url-escaped (as %3B);
this is useful when animating a (semi-colon separated) list of IRIs because
otherwise the semicolon would be interpreted as a list separator.

The five types of SVGViewAttribute may occur
in any order, but each type may only occur at most one time in a correctly
formed SVGViewSpec.

When a source document performs a link into an SVG document, for example
via an HTML anchor element
([HTML4], section 12.2; i.e.,
<a href=...> element in HTML) or an
XLink specification [XLINK], then
the SVG fragment identifier specifies the initial view into the SVG document,
as follows:

If no SVG fragment identifier is provided (e.g, the specified IRI did
not contain a "#" character, such as MyDrawing.svg),
then the initial view into the SVG document is established using the view
specification attributes (i.e., ‘viewBox’, etc.) on the
outermost svg element.

If the SVG fragment identifier addresses a ‘view’ element within
an SVG document (e.g., MyDrawing.svg#MyView
or MyDrawing.svg#xpointer(id('MyView')))
then the closest ancestor ‘svg’ element is displayed in the viewport.
Any view specification attributes included on the given ‘view’
element override the corresponding view specification attributes on the
closest ancestor ‘svg’ element.

If the SVG fragment identifier addresses specific SVG view (e.g.,
MyDrawing.svg#svgView(viewBox(0,200,1000,1000))),
then the document fragment defined by the closest ancestor ‘svg’
element is displayed in the viewport using the SVG view specification
provided by the SVG fragment identifier.

If the SVG fragment identifier addresses any element other than a
‘view’ element, then the document defined by the closest ancestor
‘svg’ element is displayed in the viewport using the view
specification attributes on that ‘svg’ element.

17.3.3. Highlighting views

It is helpful to users if the target element(s) are highlighted. The visual
styling of this highlight should be decided by the document author, because the
SVG user agent has no way to determine what changes would make the
elements more visible.

The CSS :target selector ([SELECTORS],
section 6.2.2) may be used in a stylesheet to provide alternate styling for
elements which are the target of links. For example:

<style type="text/css">
#foo:target {filter: url(#glow)}
/* when the element with id foo is linked to, use a glow filter */
.bar :target {stroke: green; fill-opacity: 0.5}
/* when any descendants of elements with class bar are linked
to, make the fill partly transparent and use a green stroke */
:target {stroke: red }
/* for everything else, just use a red stroke */
</style>

Corresponds to attribute ‘viewTarget’ on the given ‘view’
element. A list of DOMString values which contain the names listed in
the ‘viewTarget’ attribute. Each of the DOMString values can be
associated with the corresponding element using the getElementById()
method call.